1. Field of the Invention
The present invention relates generally to satellite communications and, more particularly, noise cancellation in narrowband satellite systems.
2. Description of the Related Art
Efficient use of available bandwidth in wireless, such as satellite, communications applications is a problem of paramount importance. An example of such a narrow band satellite includes very small aperture terminal (VSAT) systems. VSAT systems use compact earth stations that are installed at one or more customer""s premises to provide links among the premises over a wide coverage area. Typically, in such systems, remote ground terminals are used for communicating via a geosynchronous satellite from a remote location to a central hub station or other remote locations. The central hub station communicates with multiple remote ground terminals. VSAT systems are used to handle customer network requirements, from small retail sites up to major regional offices, and can support two-way data, voice, multimedia, and other types of data. A particular advantage of these systems is their relatively low site cost and small earth-station size.
In wireless systems, multiple users share the same bandwidth. Channel sharing through fixed-allocation, demand assigned or random-allocation modes is known as multiple access. Two of the more commonly known basic multiple-access techniques include time division multiple access (TDMA) and code division multiple access (CDMA).
VSAT type systems have traditionally implemented TDMA using time division multiplexed (TDM) mode. Such systems generally are used for low speed (300 bps to 19,200 bps) data communications such as credit card processing and verification, point-of-sale inventory control and general business data connectivity. A typical TDM/TDMA network, when implemented in a star topology (FIG. 1), uses a large satellite hub system that manages all network terminal access and routing. Data is transmitted to and from the hub in short bursts on satellite channels that are shared with a number of other VSAT terminals. The hub communicates with these VSAT terminals over a higher speed outbound TDM satellite carrier. The terminals transmit back to the hub on assigned inbound carriers using TDM protocols. Such a combination enables a predetermined number of slots in time each second that each terminal can transmit data. In addition, more or less time can dynamically be assigned to the terminals based upon each terminal""s individual requirements.
In contrast, in a CDMA type system a user""s station signal is multiplied by a unique spreading code at a high speed to be spread in a wide frequency band. Thereafter, the signal is transmitted to a transmission path. In a receiving side, the signal that was multiplexed by the spreading code is subjected to a despreading process to detect a desired signal. Signal detection is based on a unique spreading code assigned to a user""s station. If despreading is carried out with reference to a particular code used to spread a transmission signal, a user""s station signal is correctly reproduced.
Regardless of the access technique used, increased efficiency and lower cost is a primary goal. Accordingly, efficiencies in bandwidth may be realized using techniques such as crowding of adjacent channels, frequency re-use, and increasing of data rates, generally resulting in an increased amount of data traveling through the limited amount of available bandwidth. Unfortunately, however, such techniques introduce a significant amount of interference which must be canceled. Combined multi-user detection and decoding is believed to have the potential to improve performance to match that of an interference-free system. However, most development heretofore has been in CDMA based systems. In particular, it is known that the optimum receiver for CDMA system employing Forward Error Control (FEC) coding combines the trellises of both the multi-user detector and the FEC code. However, the complexity of such a receiver is exponential in the product of the number of users and the constraint length of the code. This complexity makes the use of the optimal detector prohibitive for even small systems.
Accordingly, there is a need for a lower complexity cancellation scheme in narrow band type satellite applications that allows for efficient utilization of available bandwidth by eliminating the interference resulting from the aggressive channel crowding.
Briefly, the present invention relates to a satellite communications system and method for achieving efficient utilization of available bandwidth for satellite applications such as fixed wireless, mobile satellite systems and other narrow-band type applications. A soft decision-feedback scheme is used iteratively in combination with Forward Error Correction (FEC) decoding for interference cancellation to enable efficient use of the available bandwidth through aggressive crowding of adjacent channels.
In a first embodiment of the present invention, a multiple channel decoding receiver is provided. The multiple channel receiver includes a matched-filter bank that is used to receive signals and provide initial estimates of data. The estimates are fed to an interference canceler and soft-input, soft-output decoders wherein soft interference estimates are generated by the decoders and fed back into the interference canceler. The soft estimates of the interfering signals are subtracted from the matched-filter outputs in the interference canceler to generate new, refined, soft approximations of the data. These refined soft estimates are then fed to the channel decoders for the next iteration. The process is repeated iteratively until the performance advantage is not commensurate with the computational load required to obtain it.
In another embodiment of the invention, a receiver equipped with a single channel decoder is provided. This receiver also includes a matched-filter bank that is used to demodulate multiple signals and provide initial estimates of data. The estimates are fed to a minimum mean squared error (MMSE) filter (interference canceler). The output of the interference canceler is fed to the decoder. After each decoding iteration, the decoder soft outputs are used to update the MMSE filter coefficients. The new coefficients are used by the MMSE filter to generate more accurate data estimates. The process is also repeated iteratively.